Although they are proven to be highly toxic and carcinogenic, chlorinated hydrocarbons (CHC's) are still widely used in the manufacturing of many chemical compounds, such as herbicides, fungicides and pharmaceuticals [1]. CHC's are also applied in dry cleaning processes, in degreasing operations and as organic solvents [2]. As a consequence, CHC's are found in the flue gases of many industrial installations. In the present context industry is defined broadly, it encompasses combustion processes, from power plants to municipal waste incineration, and also processes where volatile chlorinated hydrocarbons are made (both deliberately and as by-products), or where they are used. The compounds that are important for this activity and are thought to have important fluxes from industry are: carbon tetrachloride (CCl4), methyl chloride (CH3Cl), dichloromethane (CH2Cl2 or methylene chloride), trichloromethane (CHCl3 or chloroform), trichloroethene (CCl2═CHCl or trichloroethylene) and tetrachloroethene (CCl2═CCl2 or perchloroethylene), chlorobenzene, chlorotoluene as well as derivatives thereof.
The current method to remove CHC's is thermal incineration at temperatures higher than 1300° C. These high temperatures are required to avoid the formation of dioxins and polychlorobiphenyls (PCB's) [3]. Because of the high incineration temperatures and consequently, the high costs, scientists are forced to look for other but cheaper alternatives that are not harmful to the environment [4].
A first alternative process is the catalytic oxidation of CHC's at temperatures between 300° C. and 550° C. over supported noble metal catalysts (e.g. Pt, Pd and Au) [5-9]. The essential drawback here is the deactivation of the catalyst by the decomposition products including Cl2 and HCl [10]. Another disadvantage is the formation of volatile oxychlorides, which can condense and block the installation in the colder parts of the reactor. The formation of these by-products can (partially) be solved by adding small amounts of steam to the CHC's feed. In contrast, it has been stated that supported transition metal oxide catalysts are resistant to these kinds of deactivation [11]. Among these types of oxides Cr2O3 seems to be the most promising catalyst for the total oxidation of CHC's [12-15]. Frequently used supports are Al2O3, TiO2 and SiO2[16]. Other classes of materials are zeolites (e.g. H—Y and H-ZSM-5 zeolites), perovskites (e.g. LaCoO3 and LaMnO3) and pillared clays [17-21].
A second alternative for incineration is hydrodechlorination in which a CHC is transformed in the presence of hydrogen into the corresponding alkane and HCl [22,23]. Commonly used catalysts are Pd and Pt on various supports [24,25]. Ni/SiO2 catalysts also seem to possess a high activity [26]. Although this technique has economic and environmental advantages, Including the re-use of the reaction products and the elimination of hazardous by-products (e.g. Cl2 and COCl2), it is not used very often. The main reason is the very fast deactivation of the catalyst material. This deactivation is probably due to the interaction between HCl and the catalyst and to coke formation caused by oligomers formed on the acid sites of the catalyst.
A third alternative for incineration was provided by Weckhuysen e.a. (J. Phys. Chem B, 1998, 102, 3773-3778). They have studied the destructive adsorption of carbon tetrachloride on alkaline earth metal oxides, more specifically BaO, SrO, CaO and MgO. They concluded that alkaline earth metal oxides are active materials for the destructive adsorption of carbon tetrachloride in the absence of O2. The destruction activity parallels the basicity of the alkaline earth metal oxide; i.e., the activity towards CCl4 decreases in the order: BaO>SrO>CaO>MgO. Carbon tetrachloride destruction was accompanied by the formation of chlorides (BaCl2; SrCl2, CaCl2 and MgCl2 in the case of BaO, SrO, CaO and MgO, respectively). They observed that the resulting barium chloride is recycable by dissolving the solid in water, followed by precipitation and heating in oxygen. The biggest disadvantage of this technique however is that it is a stoechiometric and not a catalytic process. This means that, once the metal oxide is converted to the corresponding chloride, the activity of the system falls back to almost undetectable destruction levels.
The U.S. Pat. No. 4,561,969 provides a process for the removal of the halogen moiety from halogenated hydrocarbon feedstock. The homogenous process described in this patent depends on the use of sulfuric acid and a lanthanide oxide, the latter being required to break the chlorine ion from the hydrocarbon in order to form a chlorosulfonic acid. The oxides are regenerated by bubbling O2 through the depleted H2SO4 solution.
The present invention provides a solution to the aforesaid problems by offering methods and catalytic compositions for hydrolytic destruction of halogenated hydrocarbons in a heterogeneous process. In addition the invention provides methods to control the reaction products obtained in the catalytic process, this control allowing the conversion of chlorinated hydrocarbons towards valuable chemicals.